1. Field of the Invention
The present invention relates to a grain cleaning machine, and a method of controlling the grain cleaning machine, in which a cleaning chamber housing a rotatable cleaning roll has an outlet opened to discharge cleaned grain when an internal load of the cleaning chamber exceeds a predetermined spring load.
2. Description of the Prior Art
A conventional grain cleaning machine is constructed as shown in FIG. 10.
This machine has a grain cleaning housing 501 including a feed roll 502 and a cleaning roll 503. A baffle 504 is disposed at a grain outlet 507 of the housing 501 for restricting a sectional area of a passage through the outlet. A grain cleaning operation is carried out by rotating the feed roll 502 and cleaning roll 503 by means of a motor 505. The feed roll 502 transports uncleaned grain such as rice 506 delivered from above toward the baffle 504. The rice is cleaned between a screen 509 mounted inside the cleaning housing 501 and the rotating cleaning roll 503. Cleaned white rice is discharged through the grain outlet 507. When the grain outlet 507 is opened to a small extent, the cleaning housing 501 has an increased internal load to whiten the rice to a high degree. When the grain outlet 507 is opened to a large extent, the cleaning housing 501 has a small internal load to whiten the rice to a low degree.
Conventionally, the baffle 504 is biased by a spring 508 in a direction to close the grain outlet 507 in order to maintain a constant whitening degree. The grain outlet 507 is opened to a larger extent when the rice under treatment applies a force exceeding the spring load to the baffle 504, thereby to discharge rice whitened to a desired degree from the cleaning housing 1.
However, skill is required and operability is poor where a desired degree of whiteness is obtained by adjusting the biasing force of the spring 508.
A technique of automatically varying the biasing force of the spring 508 is known from Japanese Patent Publication No. 1989-12542. According to this technique, a current flowing to the motor 505 is detected through a current transformer, and an actuator (not shown) is provided to vary the biasing force of the spring 508 for causing the detected current level to approach a predetermined value for realizing a desired degree of whiteness.
However, the control based on the current level cannot assure high precision because of source voltage variations and variations in the characteristics of the motor 505.
Further, although the degree of whiteness is automatically controlled by the prior art technique, no means is provided for determining and displaying completion of a grain cleaning operation with a small quantity of uncleaned grain remaining in the cleaning housing 1. In practice, the motor 5 is run in spite of the cleaning operation having been completed, and is stopped only when the operator feels that the grain is sufficiently cleaned. With known grain cleaning machines, uncleaned grain fed at the start of a cleaning operation is discharged without being cleaned. To cope with such a situation, Japanese Patent Publication Kokai No. 1987-227454 discloses a grain cleaning machine including a recirculating passage specially provided for feeding a first discharge of uncleaned grain back to the machine.
The illustrated horizontal type grain cleaning machine known in the art has the spring 508 applying a constant biasing force from beginning to end of a cleaning operation. As a consequence, when the cleaning operation draws toward the end, the degree of whiteness becomes insufficient and the uncleaned grain 506 remains in the cleaning housing 501 without being discharged since the uncleaned grain is fed in a reduced amount to the cleaning housing 1 toward the end of the operation. In the case of a vertical type cleaning machine in which gravity is operable, a desired degree of whiteness cannot be obtained when the cleaning chamber has a reduced internal load toward the end of a cleaning operation. If the baffle 504 is opened to allow grain to flow through the grain outlet 507, insufficiently cleaned grain will be discharged from the machine.